Bug 6482 - allow the mysql details to be changed during koha-create

Bug 6482 - allow the mysql details to be changed during koha-create

This allows the database details (username and database name in
particular) to be modified in between a koha-create --request-db and
--populate-db. Most useful when you discover that the username is too
long or something. Now, when populating the database, the connection
information is reloaded from the koha-conf.xml rather than being
generated from the instance name.

Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>